As switching semiconductor elements for controlling relatively large currents, power devices are known. Power devices include power transistors and power MOSFETs and IGBTs and the like. Of these, IGBTs are used for example in inverters of electric vehicles as devices having the merits of ease of driving resulting from voltage drive and low loss resulting from conductivity modulating effect.
FIG. 5 shows the structure of a conventional IGBT module. This IGBT module 100 has a heat-sinking plate 104 for cooling a semiconductor element 103 having a working part 101 and a guard ring part 102. The heat-sinking plate 104 consists of a bottom heat-sinking plate joined to a substrate 105 consisting of an insulator provided below the working part 101 and the guard ring part 102. The semiconductor element 103 consists of an IGBT.
The working part 101 is made up of a high-resistance layer 106 of a first conductive type (N-type) semiconductor, a buffer layer 107 of the first conductive type (N+-type) semiconductor positioned below that, a base layer 108 of a second conductive type (P-type) semiconductor formed in an upper part of the high-resistance layer 106 of the first conductive type semiconductor, an emitter region 109 of the first conductive type (N-type) semiconductor formed in an upper part of this base layer 108, an emitter electrode 110 contacted with this emitter region 109, and a gate electrode 112 formed insulated by an insulator 111 on a channel region of the base layer 108 of the second conductive type semi-conductor.
A collector layer 113 of the second conductive type (P+-type) semi-conductor is formed on the underside of the buffer layer 107 of the first conductive type semiconductor. A collector electrode 114 is contacted with this collector layer 113.
The guard ring part 102 has a second conductive type (P-type) semiconductor layer 115 formed in an upper part of the N-type semiconductor layer 106 and an insulating film 116 of SiO2 or the like deposited on the upper part of this semiconductor layer 115. The reference number 117 is a gate electrode lead wire.
Above the working part 101 and the guard ring part 102, over a part of the emitter electrode 110 and over the insulating film 116, for leak current suppression, the surface is covered with a passivation film 118 of polyamide or the like so that the SiO2 that is the insulating film 116 does not become exposed.
The semiconductor constituting the semiconductor element 103 is made of silicon (Si). The emitter electrode 110 is made of aluminum silicon (AlSi). The collector electrode 114 is made of a metal 114a consisting of silver (Ag) or gold (Au) and a metal 114b consisting of nickel (Ni). The collector electrode 114 and the substrate 105 are joined with solder 119. The emitter electrode 110 is wired with a wire 120 made of aluminum or the like. The heat-sinking plate 104 is made of aluminum or copper or the like. The substrate 105 is joined to the heat-sinking plate 104 with solder 121.
FIG. 6 shows the structure of a conventional diode module. The diode module 200 shown in FIG. 6 has a heat-sinking plate 204 for cooling a semi-conductor element 203 having a working part 201 and a guard ring 202. The heat-sinking plate 204 is joined to a substrate 205 consisting of an insulator provided below the working part 201 and the guard ring 202. The semi-conductor element 203 is a diode.
The working part 201 is made up of a high-resistance layer 206 of a first conductive type (N-type) semiconductor, a semiconductor layer 207 of the first conductive type (N+-type) positioned below that, a semiconductor layer 208 of a second conductive type (P+-type) formed in an upper part of the high-resistance layer 206 of the first conductive type semiconductor, a cathode electrode 209 contacted with the semiconductor layer 208 of the second conductive type, and an anode electrode 210 contacted with the semiconductor layer 207 of the first conductive type.
The guard ring 202 is made up of a second conductive type (P-type) semiconductor layer 211 formed in an upper part of the N-type semiconductor layer 206 and an insulator 212 of SiO2 or the like deposited on the upper part of this semiconductor layer 211. Over a part of the cathode electrode 209 and over the insulator 212 made of SiO2, for leak current suppression, the surface is covered with a passivation film 213 of polyamide or the like so that the SiO2 does not become exposed.
The semiconductor constituting the semiconductor element 203 is silicon (Si). The cathode electrode 209 is aluminum silicon (AlSi) or aluminum/titanium nickel/titanium (Al/TiNi/Ti). The anode electrode 210 is made up of a metal 210a such as silver (Ag) or gold (Au) and a metal 210b such as nickel (Ni). The anode electrode 210 and the substrate 205 are joined with solder 214. The cathode electrode 209 is wired with a wire 215 made of aluminum or the like. The heat-sinking plate 204 is made of aluminum or copper or the like. The substrate 205 is joined to the heat-sinking plate 204 with solder 216.
When these power devices are used in an inverter of an electric vehicle or the like, a large current of several hundred amperes (A) flows and the semi-conductor elements themselves produce heat. Because of this, in conventional art a heat sink or a water-cooling mechanism has been provided on the bottom (the collector electrode side) of the semiconductor element to cool it. However, it has sometimes happened that the cells on the top face side of the semiconductor element are not cooled and cells are destroyed.
With respect to this, technology in which heat is dissipated from the upper face of the semiconductor element by a flat metal plate (strap) being provided on the upper face side of the semiconductor element and doubling as both a lead from an electrode and a heat-sinking plate has been proposed for example in JP-A-2000-124398, JP-A-2000-156439 and JP-A-2002-33445.
For example, in JP-A-2000-124398, in a power semiconductor module formed with a power semiconductor chip mounted on an insulating substrate and a metal flat plate and having an interconnecting member having an electrode-facing part facing an electrode part of the power semiconductor chip, a vertical part extending bent from this electrode-facing part, and a readout part connecting with this vertical part, a power semiconductor module is disclosed in which the electrode part of the power semiconductor chip and the electrode-facing part of the interconnection member are joined with an electrically conducting resin.
In JP-A-156439, in a power semiconductor module in which a power semiconductor element is received in a box with its underside mounted on a heat-sinking plate, a power semiconductor module is disclosed in which a flat plate form or block form heat-sinking member joined to the upper face of the power semiconductor element and to the heat-sinking plate top is provided, and heat is dissipated to the heat-sinking plate from the upper face of the semi-conductor element via the heat-sinking member.
In JP-A-2002-33445, a semiconductor device is disclosed which is made by disposing 2 or more power elements on a main frame, in which at least active faces of the power elements are connected by way of a metal frame for connection.
In a semiconductor device in which a flat metal plate (strap) is provided on the upper face side of a semiconductor element to effect heat-sinking from the element upper face like this, the semiconductor element upper face and the strap are electrically connected by an adhesive having electrical conductivity and a certain amount of thermal conductivity such as solder or an electrically conducting resin.
Because of this, in the technologies disclosed in JP-A-2000-124398, JP-A-2000-156439 and JP-A-2002-33445 in which a flat metal plate (strap) is provided on the upper face side of the semiconductor element to effect heat-sinking from the element upper face, the heat-sinking characteristics of cells formed directly below the passivation film and other cells greatly differ. When cells operate in this kind of state, current flows, joule heat is produced, and a latchup phenomenon tends to occur. When latchup occurs, locally current flows and high heat is produced, and this has been a cause of cell breakdown such as PN junction breakdown.
And, in this case, because the coefficient of thermal expansivity of the strap, which is metal, differs from the coefficient of thermal expansivity of the semiconductor element (Si) and the passivation film (polyamide, SiN, SiO, SiON, PSG (Phosphorous Silicate Glass), SiO2, NSG (Nondoped Silicate Glass) etc.) or the silicon substrate, cross-direction stresses act on the guard ring part below the passivation film due to heat production by the semiconductor element itself and thermal shocks in thermal shock testing, and cracking of the silicon substrate of the guard ring part occurs. Due to this cracking, as a result a voltage withstandability drop is brought about.
And, in conventional semiconductor elements, the passivation film has been formed as far as above the emitter electrode of the cell region, in consideration of side edges arising during pattern processing.
Accordingly, technology has been awaited for solving the problem of voltage withstandability drop resulting from stresses arising in the guard ring part due to differences in thermal expansion coefficient between the strap and the semiconductor element and the passivation film in a structure for effecting heat-sinking from the upper face of a semiconductor element, and solving the problem of cell breakdown arising because the heat dissipation characteristics greatly differ between cells directly below the passivation film and other cells.